The present invention relates to a phase synchronizing circuit for use in a multi-level, multi-phase, superposition-modulated signal transmission system in which the modulated carrier waves are mutually synchronized and, more particularly, to a phase synchronizing circuit of this type having a circuit for compensating the phase and amplitude variation of the phase error signal caused by the superposition-modulation.
Now that digital transmission has been in practical use, efforts have been made recently to enhance the transmission capacity of a carrier wave and also to enhance the efficiency with which the frequency band is used. One of the proposals made as a result of such efforts is the multi-level, multi-phase, superposition-modulated wave transmission system in which a first and a second modulated carrier waves are synthesized under synchronized state to constitute a composite carrier wave for transmission.
An ordinary multi-level, multi-phase, superposition-demodulator apparatus for such transmission system includes the so-called re-modulation type demodulator synchronized with the incoming transmitted carrier wave, and a vector subtractor circuit coupled to the demodulator.
The demodulator is adapted to demodulate at its first demodulator section the first modulated carrier wave contained in the modulated carrier wave. Also, the second modulated carrier wave is obtained by deriving the remodulated first modulated carrier wave from the first modulator section and subjecting it to vector subtraction. This second modulated carrier wave is demodulated at the second demodulator section similar to the aforementioned first demodulator circuit.
This demodulator circuit, that is, the so-called re-modulation type phase synchronizing circuit functions as a phase synchronizing circuit for the first modulated carrier wave component. However, since the second modulated wave component behaves as an interference wave, the reproduced carrier wave tends to be associated with jitters. This adversely affects the quality of the demodulation output.